Overal Heat Transfer Coefficient

Introduction and brief overview of Overall heat transfer Coeffcient

1. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE MECHANICAL DEPARTMENT 5 th SEMESTER OVERALL HEAT TRANSFER COEFFICIENT (HEAT TRANSFER– 2151909) PREPARED BY: DHEIRYA JOSHI

2. INTRODUCTION The  overall heat transfer coefficient   is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in heat exchangers, but can be applied equally well to other problems. The overall heat transfer coefficient, or U-value, refers to how well heat is conducted over a series of mediums. Its units are the W/(m 2 °C) [Btu/(hr-ft 2 °F)]. The overall heat transfer coefficient is influenced by the thickness and thermal conductivity of the mediums through which heat is transferred. The larger the coefficient, the easier heat is transferred from its source to the product being heated.

3.  In a heat exchanger, the relationship between the overall heat transfer coefficient (U) and the heat transfer rate (Q) can be demonstrated by t he following equation: where Q = heat transfer rate, W=J/s [ btu /hr] A = heat transfer surface area, m 2  [ft 2 ] U = overall heat transfer coefficient, W/(m 2 °C) [Btu/(hr- ft 2 °F)] Δ T LM  = logarithmic mean temperature difference, °C [°F]

4. To simplify the calculation, the following values may be used as a reference for the convective heat transfer coefficients: Fluid Convective heat transfer coefficient (h) Water about 1000 W/(m 2 °C) [176 Btu/(hr-ft 2 °F)] Hot Water 1000 – 6000 W/(m 2 )°C [176 - 1057 Btu/(hr-ft 2 °F)] Steam 6000 – 15000 W/(m 2 °C) [1057 - 2641 Btu/(hr-ft 2 °F)] The heat transfer coefficient has SI units in watts per squared meter kelvin : W/(m 2 K). Heat transfer coefficient is the inverse of thermal insulance . This is used for building materials (R-value) and for clothing insulation.

5. A single plate exchanger with media A transfers heat to media B. The wall thickness is  0.1 mm  and the material is polypropylene PP, aluminum or stainless steel. Media A and B are air with a convection heat transfer coefficient of  h air  = 50 W/m 2 K. The overall heat transfer coefficient U per unit area can be expressed as: U = 1 / (1 / h A  + dx w  / k + 1 / h B )           Using the values from above the overall heat transfer coefficient can be calculated to: Polypropylene PP :   U = 24.5 W/m 2 K Steel : U = 25.0 W/m 2 K Aluminum :   U = 25.0 W/m 2 K 1 W/(m 2  K) = 0.85984 kcal/(h m 2   o C ) = 0.1761 Btu/(ft 2  h  o F ) **EXAMPLE**

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